In general, in tires of the heavy goods vehicle type, the carcass reinforcement is anchored on each side in the region of the bead and is surmounted radially by a crown reinforcement consisting of at least two superposed layers formed of threads or cords which are parallel within each layer and crossed from one layer to the next making angles of between 10° and 45° with the circumferential direction. The said working layers, which form the working reinforcement, may also be covered with at least one layer known as a protective layer which is formed of reinforcing elements that are advantageously metal and extensible, known as elastic elements. It may also comprise a layer of metal cords or threads of low extensibility making an angle of between 45° and 90° with the circumferential direction, this ply, known as the triangulation ply, being situated radially between the carcass reinforcement and the first crown ply known as the working crown ply, formed of parallel threads or cords making angles at most equal to 45° in absolute value. The triangulation ply, together with at least the said working ply, forms a triangulated reinforcement which, under the various stresses it encounters, deforms very little, the triangulation ply having the essential role of reacting transverse compressive forces to which all of the reinforcing elements in the crown region of the tire are subjected.
In the case of tires for heavy goods vehicles, just one protective layer is usually present and its protective elements are usually oriented in the same direction and at the same angle in terms of absolute value as those of the reinforcing elements of the radially outermost, and therefore radially adjacent, working layer. In the case of construction plant tires intended to run over somewhat uneven ground, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the next and the reinforcing elements of the radially inner protective layer being crossed with the inextensible reinforcing elements of the working layer radially on the outside of and adjacent to the said radially inner protective layer.
Cords are said to be inextensible when the said cords under a tensile force equal to 10% of the rupture force, exhibit a strain of at most 0.2%.
Cords are said to be elastic when the said cords are under a tensile force equal to the rupture load exhibit a strain of at least 3% with a maximum tangent modulus of less than 150 GPa.
Circumferential reinforcing elements are reinforcing elements which make, with the circumferential direction, angles contained in the range +8°, −8° about 0°.
The circumferential, or longitudinal, direction of the tire is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto.
The axis of rotation of the tire is the axis about which it turns during normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
As far as the metal cords or threads are concerned, the rupture force (maximum load in N), rupture strength (in MPa) and elongation at break (total elongation in %) measurements are taken under tensile load in accordance with standard ISO 6892, 1984.
Certain present-day tires known as “road” tires are intended to run at high speed over increasingly long journeys, because of the improvements to the road network and the growth of the motorway network throughout the world. All of these conditions in which such a tire has to run undoubtedly allows an increase in the distance that the tire can cover, since tire wear is lower, but tire endurance and in particular crown reinforcement endurance is penalized.
This is because there are stresses in the crown reinforcement, particularly shear stresses between the crown layers, combined with a not-insignificant increase in the operating temperature at the ends of the axially shortest crown layer, and these cause cracks to appear and spread through the rubber at the said ends. The same problem is encountered at the edges of two layers of reinforcing elements, the said other layer not necessarily having to be radially adjacent to the first.
To improve the endurance of the crown reinforcement of the type of tire being considered here, solutions relating to the structure and quality of the layers and/or profiled elements made of rubber compound which are positioned between and/or around the ends of plies and more particularly the ends of the axially shortest ply have already been supplied.
Patent FR 1 389 428, to improve resistance to degradation of the rubber compounds located near the crown reinforcement edges, recommends the use, in combination with a low-hysteresis tread, of a rubber profiled element that covers at least the sides and marginal edges of the crown reinforcement and consists of a low-hysteresis rubber compound.
Patent FR 2 222 232, to avoid separation between crown reinforcement plies, teaches the coating of the ends of the reinforcement in a cushion of rubber the Shore A hardness of which differs from that of the tread surmounting the said reinforcement, and higher than the Shore A hardness of the profiled element of rubber compound positioned between the edges of crown reinforcement and carcass reinforcement plies.
French application FR 2 728 510 proposes positioning, on the one hand between the carcass reinforcement and the carcass reinforcement working ply radially closest to the axis of rotation, an axially continuous ply formed of inextensible metal cords that make an angle at least equal to 60° with the circumferential direction and the axial width of which is at least equal to the axial width of the shortest working crown ply and, on the other hand, between the two working crown plies an additional ply formed of metal elements directed substantially parallel to the circumferential direction.
Prolonged running under particularly arduous conditions of tires built in this way have revealed limits in terms of the endurance of these tires.
In order to address such disadvantages and improve the endurance of the crown reinforcement of these tires, it has been proposed for at least one additional layer of reinforcing elements substantially parallel to the circumferential direction to be combined with the working crown layers. French application WO 99/24269 notably proposes, on each side of the equatorial plane and in the immediate axial continuation of the additional ply of reinforcing elements substantially parallel to the circumferential direction, that the two working crown plies formed of reinforcing elements that are crossed from one ply to the next be coupled over a certain axial distance and then later uncoupled using profiled elements of rubber compound over at least the remainder of the width common to the said two working plies.
The layer of circumferential reinforcing elements usually consists of at least one metal cord wound to form a turn laid with respect to the circumferential direction at an angle of less than 8°.
The results obtained in terms of endurance and wear in prolonged road running at high speed are satisfactory. However, it would seem that the same vehicles sometimes have to drive on untarmaced roads or tracks, for example in order to get to a construction site or an unloading area. Running on such terrain is done at low speed but the tires, particularly their treads, are subject to attack for example on account of the presence of stones which greatly impair the performance in terms of tire wear.